In the context of testing semifinished products for near-surface defects, magnetic leakage flux methods are an important component for monitoring quality in the process for producing the products. Magnetic leakage flux methods are less sensitive to some disturbing properties of semifinished products, such as, for example, roughness of the surface or scale coating, than the Eddy current method or ultrasonic testing, for example. This results in a better ratio between signal used and noise signal (S/N ratio), as a result of which more reliable fault identification is made possible.
In a device for detecting near-surface defects by leakage flux measurement, a test volume of the test sample is magnetized by a magnetization apparatus and scanned with the aid of at least one magnetic-field-sensitive test probe (leakage flux probe) for the detection of magnetic leakage fields caused by the defects.
The magnetic flux generated by a magnetization apparatus in the test sample is spatially distributed substantially homogeneously in the defect-free material. Cracks or other defects act as regions of increased magnetic resistance, and so field components in the vicinity of a defect are guided around the defect and also forced out from the metal in the region near the surface. The field components forced out are detected in the leakage flux methods for detecting the defects. In a leakage flux measurement, a near-surface defect (also called surface fault) is detectable when the field components displaced from the test sample reach as far as the region of the test probe and have a field strength sufficient for the detection.
The surface faults can be classified, e.g., according to their position in the material. There are near-surface defects, which reach as far as the surface of the test sample, that is to say, for example, cracks leading from the surface to within the material or cavities open to the surface, or the like. These can be referred to as “open faults” or “visible faults.” However, there are also defects which lie concealed below a surface that appears more or less undisturbed, that is to say, for example, cracks in the depth of the material, such as stress cracks, or cracks which, although they reach as far as the surface in one production stage, have been closed again by near-surface deformation in a subsequent rolling process. These faults can be referred to as “concealed faults”; they are also referred to as “core faults” in the case of test samples composed of solid material and as “wall faults” in the case of tubular test samples.
The leakage flux test methods and test devices are subdivided, depending on how the material to be tested is magnetized, into methods and devices with constant field magnetization (DC leakage flux testing) and methods and devices with alternating field magnetization (AC leakage flux testing).
The methods with constant field magnetization are used in the testing of pipes, where both external faults, that is to say faults on the exterior side of the pipe, and internal faults, that is to say faults on the interior side of the pipe, are intended to be detected. A significant advantage of constant field magnetization is utilized here, namely the large penetration depth, such that internal faults can also be detected. In the case of very narrow and/or obliquely leading faults, by contrast, frequently only unsatisfactory test results are obtained.
A significant advantage of leakage flux methods with alternating field magnetization is the very high resolution for extremely small faults on the outer surface, that is to say for open faults. Therefore, the alternating field magnetization is generally employed when only exterior faults are intended to be detected, which is frequently the case for example with solid material such as round billets, square billets or bar steel. What is disadvantageous is that, as a result of the small penetration depth of the alternating field, deep faults which are not open to the surface (concealed faults) frequently can only be identified unsatisfactorily or cannot be identified at all.
DE 10 2004 035 174 B4 describes a method and a device for the nondestructive testing of pipes composed of ferromagnetic steel by leakage flux, wherein the pipe (test sample) is magnetized by a constant field. To enable better fault assignment between pipe outer surface and pipe inner surface, the amplitude of the horizontal field component of the magnetic leakage flux, the amplitude varying in the vertical direction, is detected firstly at a near-surface distance from the pipe outer surface and secondly at a distance further away from that and the detected signals are related to one another by means of difference formation, wherein the amplitude of the vertical field component of the magnetic leakage flux is also detected in addition and related to the amplitude of the horizontal field component measured at the near-surface distance and/or at the distance further away from that.
DE 10 2006 019 128 A1 describes a leakage flux measuring instrument for detecting near-surface and surface-distant defects on ferromagnetic test samples by leakage flux measurement, wherein the test sample is likewise magnetized by a magnetic constant field. To have the effect that the faults concealed below the material surface of the test sample can be detected better by means of leakage flux observation without reducing the sensitivity for near-surface faults, a combination of at least one flat coil or probe and at least one coil or probe oriented perpendicularly thereto is provided on the test probe side.
It could therefore be helpful to provide a method for detecting near-surface defects by leakage flux measurement and also a corresponding device which make it possible to detect both faults open to the surface and concealed faults with high sensitivity.